A glow plug used for assisting in startup of a diesel engine includes a ceramic heater configured such that a heat generation resistor formed from an electrically conductive ceramic is embedded in a substrate formed from an insulating ceramic. In general, the heat generation resistor of the ceramic heater has the shape of the letter U, and a portion of the heat generation resistor corresponding to the turn portion of the letter U has a reduced diameter. Therefore, this portion functions as a heat generation portion. Further, a forward end portion of the substrate is formed into a semispherical shape which corresponds to the U-like shape of the heat generation portion. Therefore, the ceramic heater can efficiently transmit the heat generated by the heat generation portion to the outside of the substrate.
In recent years, in order to improve the startability of an engine and reduce NOx contained in exhaust gas, there has been a demand for increasing the rate of temperature rise after start of energization of a ceramic heater. In order to enhance the performance of a ceramic heater for raising temperature quickly (hereinafter referred to as “quick temperature raising performance”) without changing the design of the heat generation resistor itself or changing the magnitude of current supplied to the heat generation resistor at the time of energization, the ceramic heater is desirably designed such that the heat generated by the heat generation portion is quickly transmitted to the outside of the substrate. A possible design which allows quick heat transmission is such that the outer diameter of the ceramic heater is made smaller than that of a conventional ceramic heater so as to dispose the heat generation resistor at a position closer to the outer surface of the substrate. However, merely decreasing the outer diameter of the ceramic heater may result in breakage of the ceramic heater. Therefore, a forward end portion of the substrate is desirably tapered such that the outer diameter of the ceramic heater decreases in a region corresponding to the heat generation portion (see, for example, Patent Document 1).